Charged-particle-beam (CPB) pattern-transfer methods (such as electron-beam microlithography) take advantage of the high-resolution imagery available with electron beam optical systems. However, the throughput achieved with conventional CPB methods is low.
So-called "hybrid" pattern-transfer methods, known as "cell-projection," "character-projection," or "block exposure" are useful for transferring patterns containing large numbers of repeating subunits (each measuring, for example, about 5 .mu.m by 5 .mu.m). In hybrid pattern-transfer methods, one or more of the subunit patterns is defined by a mask and repeatedly transferred onto the wafer with a projection-exposure process. Such methods are particularly well suited for pattern transfer for the highly repetitive patterns of high-density memory chips (e.g., DRAMs). Hybrid pattern-transfer methods used in the production of a semiconductor integrated circuit (such as a DRAM) permit throughputs that are about one tenth the throughput achievable using other pattern-transfer methods.
Unfortunately, hybrid pattern-transfer methods improve throughput only for transferring patterns having significant repetition. For non-repetitive patterns, throughput is not improved because of the long times required to transfer the non-repetitive pattern portions to the wafer.
In so-called "divided-field" pattern-transfer methods, an entire circuit pattern for a semiconductor die is defined by a mask. An electron beam (or other CPB) irradiates an area of the mask and an electron-optical system projects a demagnified image of the irradiated area onto a wafer. The electron-optical system is typically a two-stage projection-lens system. Because the electron beam does not irradiate the entire circuit pattern simultaneously, the circuit pattern is divided into a number of fields or "stripes", and each stripe is further divided into sub-fields. The sub-fields are sequentially irradiated with the electron beam and imaged onto the wafer. The patterns from the sub-fields must be accurately connected ("stitched") on the wafer so that the entire die pattern is transferred. To control aberrations, the electron-optical system is adjusted for each sub-field. This method is discussed in, for example, U.S. Pat. No. 5,260,151 and Japanese laid-open patent document Hei 5-160012. Divided-field pattern-transfer methods permit excellent aberration control, but the commercial application of such methods in the fabrication of ULSI semiconductor devices is limited by their throughput.
A particularly important problem for these divided-field pattern-transfer methods is the precise stitching of the images of the mask sub-fields on the wafer. One method for connecting sub-field patterns is the "gray-splicing" method described in Japanese laid open patent document Sho 63-1032. In the gray-splicing method, mating edges of two sub-fields projected adjacent on the wafer have the same patterns and the patterns are projected to overlap on the wafer. The area of the wafer in which the patterns from the two sub-fields overlap receives two exposures, corresponding to the exposures of each of the two sub-fields. The dose (total charge per unit area) delivered to the wafer in each exposure is adjusted so that the total dose of the two exposures in the area of overlap equals the dose delivered to single-exposed areas of the wafer. Unfortunately, the gray-splicing method is applicable only to pattern-transfer methods using a variably shaped CPB or a focused CPB beam.
Idesawa et al. describe tapering the dose distribution of exposures made in a feature-connection area located between two adjacent sub-fields on the wafer. See Idesawa et al., "Discontinuity Reduction Method in Pattern Connection," J. vac. Sci, Technol., 19 (4), November/December 1981, p. 983. This technique produces a constant dose over the entire wafer with inconspicuous stitching of patterns between the mating sub-fields. Unfortunately, however, this technique is applicable only when the beam shape is variable. In addition, tapering the dose distribution is difficult.
Another problem of the divided-field method is the difficulty of accurately stitching together the images of adjacent patterns on the wafer in the presence of distortion in the pattern-transfer apparatus optical system. This distortion problem is illustrated in FIG. 7. Images 498, 499 of respective square sub-fields on the wafer exhibit pincushion distortion caused by the optical system of the pattern-transfer apparatus. (The images can also exhibit barrel distortion.) Respective corners 498a, 499a of the distorted images 498, 499 overlap as imaged on the wafer, and the overlapping region is therefore overexposed by the pattern-transfer apparatus. In addition, because the images 498, 499 are distorted, a region 497 between the images 498, 499 remains unexposed. The unexposed region 497 can cause circuit defects such as short circuits or open circuits while the double exposure at the corners 499a, 499b can produce circuit features that are larger or smaller than intended.
One solution to these problems is described in Japanese laid open patent document Hei 7-2098550 and corresponding U.S. Pat. No. 5,523,580. With reference to FIG. 8, pattern line segments 500, 510 are defined by patterns in respective sub-fields. The pattern line segments terminate with respective end portions 501, 511. The pattern line segments 500, 510 and the respective end portions 501, 511 are imaged onto the wafer so that the end portions 501, 511 overlap and a continuous line is formed.
The end portions 501,511, define a checkerboard pattern with transmitting sections such as exemplary squares 501a, 501b (shown in FIG. 8 as squares with hatch marks) and absorbing sections such as exemplary squares 501c, 501d (shown in FIG. 8 without hatch marks). When the images of the two adjacent line patterns are projected on the wafer, the end portions 501 and 511 overlap on the wafer, so that the transmitting sections of the end portion 501 coincide with the absorbing sections of the end portion 511, and vice versa. Thus, the checkerboard patterns formed on the end portions 501 and 511 are complementary. If the images of the sub-fields are accurately projected onto the wafer without distortion, the images form a single continuous straight line with no overexposed or underexposed regions because of the patterns of the end portions 501, 511 are complementary Another prior-art solution is illustrated in FIG. 9. Pattern lines 600, 610 are defined by respective sub-fields and include triangular end portions 601, 611. (The hatched areas in FIG. 9 represent transmitting sections) The pattern lines 600, 610 are ideally projected to form a single straight line.
These prior-art methods have several disadvantages. Individual squares such as exemplary absorbing squares 501c, 501d of the checkerboard pattern 501 are almost completely surrounded by transmitting sections such as squares 501a, 501b. Thus, the transmitting section 501a is not adequately supported and additional support must be provided. For example, a two-layer mask construction can be used in which an absorbing material is supported by a transmitting layer. However, fabrication of such a two-layer mask is difficult and the exposure contrast obtained with such a mask is less than that of a single-layer mask. In addition, the transmitting supporting layer is heated by the charged-particle beam.
Referring again to FIG. 9, the triangular end portions 601, 611 are not complementary. Accordingly, if the images of pattern lines 600, 610 (and the triangular end portions 601, 611) are offset due to distortion or misalignment, short circuits, open circuits or other defects arise. Even if the images are precisely connected, the wafer is overexposed in areas corresponding to the triangular end portions 610, 611.